The field of the invention pertains to threaded plugs for insertion in threaded boreholes such as the borescope holes formed in the metal casings of turbo-jet, fan-jet and other types of gas turbine engines used in aircraft. The borescope plugs, in particular those used in the hot section of the engine, must be corrosion resistant at high temperatures and not subject to loosening despite repeated temperature cycling when the engine is started and stopped. As a result, the borescope plugs are typically formed of high temperature refractory metal alloys that are difficult to form and therefore expensive to manufacture.
U.S. Pat. No. 2,166,686 discloses an adjusting screw for a camshaft follower. The screw is slotted and flatted at the end of the screw. The threads at the end of the screw are spaced by a relieved peripheral area from the remaining threads on the screw, and the lead of the end threads differs from the lead of the remaining threads.
U.S. Pat. No. 3,351,966 discloses a slotted fastener and method of manufacture of the threads to prevent the thread cutting or tapping effect adjacent the slot. The patent also discloses an enlarged thread area in the region of the slot to cause deformation when threaded into a threaded socket.
U.S. Pat. Nos. 4,815,276 and 5,115,636 disclose borescope plugs for double walled constructions, and U.S. Pat. No. 5,169,270 discloses a slotted plug wherein the slotted threaded portion is squeezed by the insertion tool when the plug is screwed into the threaded socket. Upon release of the tool, the plug expands to lock.
U.S. Pat. No. 5,704,748 discloses a screw having a recess intersecting the threads and a slot for a coil spring. The coil spring deflects upon mating with the threaded female member.